1. Field of the Invention
This invention relates to integrated circuit packages, and more particularly, to improved methods for attaching a heatspreader to an integrated circuit mounted on a substrate.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Until recently, the ongoing quest of the semiconductor industry to further improve the performance of integrated circuits (IC""s) was little impacted by packaging. However, advancements in the performance of IC""s are now being limited by the current packaging technology. Therefore, the efficient packaging of the IC""s, e.g., increased package density or the miniaturization of packaging, will play an increasingly major role in the semiconductor industry.
There are several packaging technologies developed to connect IC""s to a substrate, namely, wirebond, tape automated bonding (TAB), and flip-chip attach. The drive for faster clock speeds and more densely integrated circuits has led to integrated circuits that have a higher density of input/output (I/O) pads per chip. Flip-chip package technology such as direct chip attach (DCA), where the chip is directly attached to the board, has evolved to handle these higher I/O density chips providing manufacturing solutions such as: small package dimensions, high clock speed, low costs, and suitability for high volume production.
Instead of having the I/O pads on the periphery of the IC device as in the case, e.g., of wirebonded packaging, the I/O pads of an IC for a flip-chip application are typically arranged in a two-dimensional array upon a xe2x80x9cfrontsidexe2x80x9d surface of the IC. A flip-chip attachment method involves inverting the IC so that the xe2x80x9cfrontsidexe2x80x9d surface with I/O pads faces downward on to a substrate such as a printed circuit board (PCB) or multichip module (MCM), which has corresponding set of bonding pads. A solder bump is formed upon each of the I/O pads of the IC, and during the flip-chip mounting of the IC to the substrate, the solder bumps are placed into physical contact with the bonding pads of the substrate. The solder bumps are then subjecting to heating long enough for the solder to flow. When the solder cools and hardens, the I/O pads of the IC are electrically and mechanically coupled to the bonding pads of the substrate. This configuration is also referred to as a xe2x80x9cdie-downxe2x80x9d configuration, since the IC, or die, is facing downward toward the substrate. In the case of a peripheral-terminal device, it would be mounted face upward, which is referred to as a xe2x80x9cdie-upxe2x80x9d configuration, as in the case of wirebonding technology. xe2x80x9cDie-upxe2x80x9d mounting when compared to a xe2x80x9cdie-downxe2x80x9d configuration may be more advantageous in some regards such as by allowing improved thermal contact between the IC and the grid array substrate. However, it appears less advantageous for packaging high density circuits that require a smaller area of substrate per IC chip, and faster (shorter path) electrical connections which have less resistance, and less parasitic inductance.
There are two principal thermal problems that must be addressed in a flip-chip or xe2x80x9cdie-downxe2x80x9d configuration. One is mechanical strain on the solder bump attachments during temperature cycling due to the coefficient of thermal expansion (CTE) mismatch of the substrate with the IC. This problem has been effectively remedied by employing an underfill material, which is typically a thermosetting polymer (e.g., an epoxy resin) that is dispensed in liquid form between the IC and the substrate, and encapsulates the solder bumps. The liquid underfill material then becomes substantially rigid during a curing process, e.g., time and/or elevated temperature.
A second thermal problem that confronts flip-chip packaging is that it must provide an additional means for heat dissipation of the IC""s during use, since the IC""s are not as directly in contact with a substrate as in the xe2x80x9cdie-upxe2x80x9d configuration. Typically, a heatspreader is a flat piece of copper or some other type of heat conducting material, which is attached by a thermosetting polymer (e.g., an epoxy resin) to the xe2x80x9cbacksidexe2x80x9d surface of the inverted IC. The typical heatspreader""s lateral dimension is on the order of twice the size of the IC, and the heat it dissipates is about 5-30 watts per chip depending on the application.
One problem that arises when attaching a heatspreader to an IC is that since the heatspreader lateral dimensions will extend beyond the lateral sidewalls of the IC (i.e., the heatspreader is xe2x80x9coversizedxe2x80x9d in comparison with the IC), then the heatspreader can be susceptible to forces such as torque, which can put undue mechanical stress and strain on the attachments of the package. Typically, metal stiffener rings have been employed to alleviate any torque problems that may arise from attaching an xe2x80x9coversizedxe2x80x9d heatspreader to the back of a flip-chip attached IC. A stiffener ring is essentially a rectangular frame, though other shapes may be used, of copper or some other conducting material that laterally surrounds the IC. Typically, the stiffener ring is attached to the bottom surface of the outer edge of the heatspreader and the upper surface of the substrate by an attach epoxy. Therefore, the stiffener ring lends mechanical support to the lateral edges of the heatspreader, and remedies any undue torque problems that may result from the use of an xe2x80x9coversizedxe2x80x9d heatspreader in the IC package.
However, e.g., in the case of multichip module (MCM) packages where there are multiple IC""s mounted onto a single module, the question of available space becomes very important for high density applications. In the case of a high density IC application there are several competing concerns. In general, one wants to make the individual xe2x80x9coversizedxe2x80x9d heatspreaders as large as possible to dissipate heat from the IC""s. On the other hand, one wants to make the xe2x80x9coversizedxe2x80x9d heatspreaders as small as possible to save space and increase packing density, since the heatspreaders"" lateral dimensions are a limiting factor in the available packaging surface area on the module substrate. In a typical configuration, some minimum amount of space is required to dispense the underfill around the IC, and also one must allow for some additional space for fillets of underfill on the edges of the IC that protrude outward, which add further support to the structure. Furthermore, there has to be minimum of space allotted for the placement of the stiffener rings with a stiffener ring adhesive and also for fillets at the sides of the stiffener ring that protrude outward and provide additional support. Also for purposes of maintaining mechanical integrity of the package structure typically one allows for some finite separation distance between the fillets on the sides of the IC and the fillets on the inner sides of the stiffener ring. In some high density applications there simply may not be enough space available on the module substrate to accommodate this type of heatspreader packaging configuration.
It would be beneficial to have a method for packaging an IC, which is flip-chip mounted to a substrate, by attaching a heatspreader to a flip-chip IC package without requiring the use of a stiffener ring between the heatspreader and the substrate, while still maintaining a robust mechanical support between the substrate and the heatspreader to eliminate any torque that may otherwise arise.
The problems outlined above may be in large part addressed by the present method for packaging an integrated circuit which is flip-chip mounted to a substrate, where the substrate may be, for example, a printed circuit board or alternatively a packaging substrate such as a multichip module substrate. In an embodiment, a first liquid encapsulant, preferably a thermosetting polymer such as an epoxy underfill, may be introduced on an upper surface of an IC, which is flip-chip mounted to a substrate. A heatspreader, having planar dimensions larger than the IC, is arranged over the IC and the first liquid encapsulant, where the lateral edges of the heatspreader extend beyond the lateral edges of the. IC. Furthermore, the first liquid encapsulant is introduced between the lateral edges of the heatspreader and the substrate, where the first liquid encapsulant laterally surrounds the IC. Preferably, the first liquid encapsulant may include thermally conductive material. The first liquid encapsulant may also include electrically conductive material, and thus provide an effective grounding for the floating heatspreader. The first liquid encapsulant material may be subjected to a curing process, e.g., time and/or elevated temperature, which causes the encapsulant material to become substantially rigid, and thus form a reliable mechanical and thermal attachment of the heatspreader to the substrate.
In an embodiment, the heatspreader is attached to the integrated circuit with a thermally conductive material, preferably an adhesive epoxy. The thermally conductive material may be subjected to a curing process. In another embodiment, a second liquid encapsulant is introduced between the integrated circuit and the substrate. The second encapsulant material may be subjected to a curing process. The thermally conductive material, the first liquid encapsulant, and the second liquid encapsulant may be subjected to curing processes individually, or they may all be subjected to a single curing process together, or may be cured using two curing processes for the three thermosetting polymer materials.
In an embodiment, both the first liquid encapsulant and the thermally conductive material are of similar composition and may further have the same composition. In another embodiment, the first liquid encapsulant, the second liquid encapsulant, and the thermally conductive material are all of similar composition, and may further have the same composition.
In addition to the method discussed above, an integrated circuit structure is contemplated herein. In an embodiment the integrated circuit structure includes a heatspreader arranged over an integrated circuit flip-chip mounted to a substrate. A first continuous fill material, preferably an epoxy underfill, laterally surrounds the integrated circuit and extends vertically between the heatspreader and the substrate, such that the heatspreader is mechanically supported by the fill material. The first continuous fill material may include thermally conductive material. The first continuous fill material may also include electrically conductive material, and thus provide an effective grounding for the heatspreader.
In an embodiment, a thermally conductive material, preferably an adhesive epoxy, resides between the heatspreader and the integrated circuit. In another embodiment, a second continuous fill material, preferably an epoxy underfill, resides between the integrated circuit and the substrate.
In an embodiment, both the first continuous fill material and the thermally conductive material are of similar composition and may further have the same composition. In another embodiment, the first continuous fill material, the second continuous fill material, and the thermally conductive material are all of similar composition, and may further have the same composition.